how to show reflection of a different object in cycles
I slapped together a quick example. In short - 
The visibility options for the first object (Suzanne) have glossy, shadow and diffuse turned off. 



The second object (cone) has only visibility for glossy enabled.



This results in Suzanne not being visible in the reflection and the cone just being visible in the mirror.



Depending on what you're needing, there might be more involved ways of doing it via compositing, but I hope this answers your original question.